The interaction of Pneumocystis carinii with phagocytic cells and antipneumocystis opsonic antibody will be evaluated in vitro using techniques developed during the study of Mycoplasma pulmonis. Pneumocystis multiply in vivo associated with rodent alveolar macrophages, however, they appear to multiply extracellularly, not unlike mycoplasmas. Antibody may play an important role in control of the disease, however, the interaction of opsonins and alveolar macrophages has not been examined. Rat alveolar macrophages from normal and corticosteroid treated rats will be cultivated in vitro and examined for pneumocystis-macrophage interaction. The ingestion of pneumocystis from corticosteroid treated rats by rat alveolar marcrophages and mouse peritoneal macrophages with and without opsonins will be examined. The role of lymphocyte-macrophage interaction in control of pneumocystis infection will also be examined. Since extensive information on the dynamics of mycoplasma-macrophage interaction has been obtained, this microbe will be observed in these same systems as a control of opsonin-phagocyte interaction.